1. Field of the Invention
The present invention relates to the field of electrochromic systems, notably panes in which the light transmission can be modulated by means of an electric current. More particularly the invention relates to the control of the solar input into buildings or the passenger compartment of vehicles.
2. Discussion of the Background
It is well known that an electrochromic system comprises a film of a material capable of inserting cations in a reversible manner and the oxidation states which correspond to the inserted and de-inserted states are of distinct colorations, one of the states being generally transparent. The insertion or de-insertion reaction is controlled by the application of a suitable potential difference. The electrochromic material, normally based on tungsten oxide, is brought into contact with an electron source such as a transparent electrically conducting film, and a cation source such as an ion-conducting electrolyte.
Furthermore it is known that, in order to ensure that the electrochromic device has the capability of at least about a hundred switching operations, there must be associated with the electrochromic material film a counter-electrode which is capable, itself of reversibly inserting cations, symmetrically disposed with respect to the film of electrochromic material so that, macroscopically, the electrolyte appears as a simple medium for the cations.
The counter-electrode should be composed either of a film that is neutral in coloration or of a film that is less transparent when the electrochromic film is in the de-colored state. Since tungsten oxide is a cathodic electrochromic material, that is to say its colored state corresponds to the most reduced state, an anodic electrochromic material such as nickel oxide or iridium oxide is generally used for the counter-electrode. It has also been proposed to use a material that is optically neutral in the critical oxidation states, such as for example, cerium oxide or organic materials such as electron conducting polymers of the likes of polyaniline, or Prussian blue.
Such systems make possible a real reversibility of the insertion/de-insertion phenomena, due to the ion source constituted of the counter-electrode.
Nevertheless, it has become apparent that such systems degrade and that, after a certain time, the phenomenon of electrochromism no longer takes place or at least is very clearly weakened, which leads to a reduction or complete suppression of the contrast between the colored state and the de-colored state.
This degradation is demonstrated in three different types of tests:
i) exposure of a system in the de-colored state to ultraviolet rays;
ii) exposure of the system to heat, for example to a temperature of the order of 80.degree. C.; and
iii) cycling, a test during which the system is subjected to a very large number of alternating colored and de-colored states.
Surprisingly, for these three tests, the final result is principally the same, although each has a different relative importance and the analysis of the degraded system shows that the degradation essentially results from an irreversible reduction of the counter-electrode, generally to metal which is a non-electrochromic material. This phenomenon is common to practically all of the electrochromic devices based upon polymeric electrolytes, for a very simple reason which will become apparent further on, which conduct protons or lithium ions, the ultraviolet exposure test being the most degrading in the case of the lithium systems.
Although the final result is the same, this may nevertheless not be true for the mechanisms leading to it. Furthermore, the present inventors have found that these degradations are greatly retarded if a barrier film or layer is inserted between the electrolyte and the counter-electrode; in particular a film of LiNbO.sub.3 between a lithium electrolyte and a counter-electrode of LiNiO.sub.y or a film of tantalum oxide Ta.sub.2 O.sub.5 or of niobium oxide Nb.sub.2 O.sub.5 between a protonic electrolyte and a counter-electrode of H.sub.x IrO.sub.y.
The barrier film interposed between the electrolyte and the counter-electrode should be open to the cations of the electrolyte which are protons or cations of alkali metals which it is desired to insert or de-insert into and out of the two films of electrochromic materials. Moreover, the barrier film is preferably transparent in order not to reduce the light transmission of the system and should have a good chemical resistance with respect to the electrolyte, which implies, notably, a good resistance in acid medium in the case of the protonic systems, and in neutral medium in the case of the lithium systems.
On the other hand, this barrier film is preferably a filter that opposes the passage of ionic or gaseous species transported by the electrolyte, with the exception, of course, of the cations, and more especially a filter of the anions and the gases that may be present in the system, which are imprisoned during the assembly or formed as a consequence of parasitic degradation reactions.
In addition, this barrier film is preferably such that it increases the formation overvoltage of reducing agents at the interface between the electrolyte and the counter-electrode of anodic electrochromic material.
Finally, the barrier film is preferably an electron isolating film.
In order to understand what may be the role, or more exactly the roles, of this barrier film, it is advantageous to analyze the phenomena that are likely to occur during the course of these three different tests. In order to assist in clarifying the explanation, reference is made to a system containing an electrode based upon tungsten oxide WO.sub.3 and a counter-electrode of iridium oxide H.sub.x IrO.sub.y, but this explanation may be generalized to all the electrochromic materials with anodic coloration.
When the system is subjected to the UV test in an open circuit, bluing is first observed in the de-colored states, or in other words, cations are inserted into the film of tungsten oxide. Such a bluing is not observed in the other two tests. Nevertheless, this bluing is perfectly reversible and it is only necessary to subject the system to a normal decoloration voltage to cause this blue tint to disappear, thereby restoring the system to one having a contrast as good as the initial contrast.
It is known that WO.sub.3 is a semiconductor material of type n, that is, upon the conduction of electrons. When the semiconductor is irradiated the electrons may pass through the conduction band and thus facilitate oxidation reactions. In contrast, iridium oxide, like the majority of the counter-electrode materials of anodic coloration, is a semiconductor of type p, which conducts by holes, with the result that when the counter electrode is exposed to photons, the effect is to facilitate reduction reactions. By subjecting the WO.sub.3 /HIrO.sub.2 system to ultraviolet radiation, oxidation reactions may be expected for WO.sub.3 and reduction reactions for the counter-electrode.
No electrochromic system is perfectly anhydrous. However, many precautions are taken to eliminate every trace of water from such systems. Thus, in the preparation of an electrochromic device the WO.sub.3 film is hydrated during the course of its deposition and/or by handlings of the film before assembling which normally pass the film through open air, which is of necessity always humid. Furthermore, the proton or lithium electrolytes always have a residual humidity content, which may be maintained with difficulty at less than a few 100 ppm. The most probable oxidizing agent at the WO.sub.3 side of the system is residual water. What results is essentially a battery, which exhibits the following equilibrium reactions: EQU WO.sub.3 +xM.sup.+ +xe.sup.- .revreaction.M.sub.x WO.sub.3 ( 1) EQU x.sub./2 H.sub.2 O.revreaction.x.sub./4 O.sub.2 (adsorbed)+xH.sup.+ +xe.sup.- ( 2)
with the net equation being: EQU WO.sub.3 +x.sub./2 H.sub.2 O.sub.x M'.revreaction.M.sub.x WO.sub.3 (blue)+x.sub./4 O.sub.2 +xH.sup.+ ( 3)
where M is either H or Li.
The zero current potential of equilibrium (2) is shifted towards the negative potentials by reason of the photosensitivity of tungsten oxide, so that the reduction of WO.sub.3 to M.sub.x WO.sub.3 (M=H, Li) is favored. Conjointly, oxygen is generated on this same electrode.
It is hypothesized that the quantity of oxygen thus formed is sufficiently small and it is partially absorbed by the polymeric electrolyte, which explains the absence of the formation of bubbles, at any rate in the short term.
In practice, a stable coloration state is reached only after a few hours, and its degree of intensity at equilibrium depends upon the wavelength of the radiation. This radiation is effective in the near UV for wavelengths corresponding to the visible. It is therefore possible to limit the bluing, but not to suppress it entirely, by means of a UV filter.
In any case as indicated above, it is not at the tungsten oxide side but at the iridium oxide counter-electrode side that the system degrades irreversibly. The colored state, that is the inserted state, corresponds to an oxidation degree III. The degraded state indicates a passage to degree 0, but nevertheless the equilibrium between degree III and degree 0 is described in the literature as very improbable.
In the case of the protonic systems, it has been proposed that the following equilibriums exist in a battery situation; EQU H.sup.+ +e.sup.- .revreaction.1/2H.sub.2 ( 4) EQU HIrO.sub.2 (III).revreaction.IrO.sub.2 (IV)+e.sup.- +H.sup.+( 5)
with the net reaction being: EQU HIrO.sub.2 (III).revreaction.1/2H.sub.2 +IrO.sub.2 (IV) (6)
This reaction is followed by the reduction of Ir (IV) to Ir (O), for example by hydrogen: EQU IrO.sub.2 (II)+2 H.sub.2 .revreaction.Ir(O)+2 H.sub.2 O (7)
The net reaction of reactions (6) and (7) is: EQU 4HIrO.sub.2 (III).revreaction.3IrO.sub.2 (IV)+Ir(O)+2 H.sub.2 O(8)
It should be noted that the equilibrium reactions proposed here are probably not the only ones which occur, particularly since the measured ratio Ir(IV)/Ir(O) appears to be much less than 3. It is therefore possible that the reduction of iridium oxide to the 0 valent state is attributable also to reducing agents other than hydrogen, or that the latter is formed also by other routes. Polymeric degradation products in particular are capable of playing a part in these mechanisms.
In the case of systems containing lithium, with a counter-electrode of the AO.sub.y type, wherein A is, for example, Ni, Ir or V, it is probably possible also to write the following equilibrium reactions: EQU H.sub.2 O+e.sup.- .revreaction.1/2H.sub.2 +OH.sup.- ( 9) EQU H.sub.2 +AO.sub.x .revreaction.AO.sub.x-1 +H.sub.2 O (10)
Apart from this, the mechanisms that are proposed appear also to be applicable basically to the systems that rely upon the insertion/de-insertion of alkali metal cations. It is then possible to rewrite all the above equilibrium reactions. A need continues to exist for an electrochromic device which exhibits improved stability.